In processes where amines are converted to isocyanates using phosgene, as is well known in the art, a disadvantage is that in the phosgenation process typically an excess of phosgene is used, or that the phosgenation typically does not consume all phosgene in the reaction mixture. Hence typically the reaction mixture, either gaseous or liquid and possibly containing solids, e.g. all components present in a solvent, comprises phosgene. In the process of conversion of an amine group to an isocyanate group also hydrogen chloride is obtained and found in the reaction mixture.
Often the hydrogen chloride is separated from the reaction, and can be used in other processes or as a feed to a chlorine recycling process, provided the hydrogen chloride is purified to meet the needs of its further use.
Often there is also a need to remove the phosgene from the reaction mixture so that the phosgene can be reused.